微粒子化Ni-Co-Mn-Snメタ磁性形状記憶合金の磁性と応用

抄録

Ni-Mn-X (X = In, Sn and Sb) based ferromagnetic shape memory alloys have been intensively investigated. Because their martensitic transformation can be controlled not only by temperature but also magnetic field and stress, these alloys are thought to be good candidate for applicable materials. However, the polycrystalline specimens are difficult to use for the application because of the considerable brittleness. One of the methods to improve the brittleness is the powder metallurgy. In the present study, mechanical and magnetic properties of Ni-Co-Mn-Sn specimens fabricated by spark plasma sintering (SPS) were investigated.

Ni₄₃Co₇Mn₃₉Sn₁₁ (at.％) alloy was melted by high frequency induction and powders were obtained with using conventional nitrogen gas atomization. Powders with a diameter between 25 and 63 μm were selected, and specimens were obtained by SPS technique at 1073 or 1173 K for 15 min. The microstructures of the specimens were examined by optical microscopy. The magnetization was measured by a superconducting quantum interference device magnetometer. The mechanical properties were examined by compressive test.

Microstructural observation revealed that the sintering is hardly proceeded in the specimen sintered at 1073 K, on the other hand, sintering at 1173 K is sufficient to obtain a specimen with high density. From the magnetic measurements, change of the magnetization is observed associated to the martensitic transformation, in addition, the martensitic transformation temperature decreases about 21 K by applying the magnetic field of 7 T. For the stress‒strain curve at room temperature, the fracture occurred at a strain of about 13％ and a large plastic deformation of about 7.7％ is obtained in the sintered specimen, while the fracture started at a low strain of about 1％ for the polycrystalline bulk specimen with same composition. It can be said that the ductility was drastically improved without any large loss in the martensitic and magnetic properties.